Considerable attention is being devoted to weight reduction in automotive design in order to conserve energy. In the case of automobile bumpers, it is necessary to reduce weight and meet governmental bumper impact standards without sacrificing durability, appearance, cost-effectiveness and styling versatility of nickel-chromium plating.
High strength aluminum alloys present one promising avenue to reducing the weight of bumper systems and at the same time meet the governmental bumper impact standards. However, is it not possible, prior to this invention, (1) to commercially electroplate a consistently adherent metal coating directly on aluminum, particularly a brass coating, and (2) to provide a bright lustrous coating system for an aluminum substrate which experiences minimal lateral corrosion. This is due to several problems among which include the natural oxide film that is present upon aluminum and the interference caused by such oxide film in achieving a sound adherency between any plated material over the aluminum base. If the natural oxide film is somehow removed and replaced by a plated system, the natural corrosion resistance of aluminum is sacrificed and the plated materials become a potential galvanic couple in a corrosive environment. In such a couple, aluminum will become the anode and will tend to dissolve. Since aluminum is more reactive than steel, the dissolving rate can actually be faster than with steel bumpers.
The performance of plated aluminum can be influenced by the pre-plating treatment or underlayment system, both referred to hereinafter as pretreatment. Over the years, a number of pretreatments have been proposed, most directed to the problem of achieving high adherency. Only a few have been successful and these only to a small degree. They include (a) a chromium on nickel on copper on zinc on aluminum system (typically referred to as the zincate process), (b) a chromium on nickel on bronze on tin on aluminum system (typically referred to as the Alstan process), (c) a chromium on brass (high zinc content) an aluminum system (referred to as the Dupont process), a chromium on nickel on an immersion zinc layer (which is dissolved to some degree during immersion in the nickel bath) on aluminum system (referred to as the Alcoa 661 process), and (d) a phosphoric acid anodizing treatment wherein a chromium on nickel on anodic oxide on aluminum system is employed. Each of these systems are deficient either because they do not achieve proper adherency or they create excessive galvanic couples which accelerate corrosion between the various elements of the system.
Most of the prior art top-coat systems of decorative use here, such as nickel and chromium, were developed for use on mild steel substrates and have found particular utility therein; the top-coat systems were subsequently transplanted for use with aluminum in the hope that their performance would be comparable. However, it must be recognized that different physical parameters do exist when a plating system is applied to aluminum. An electromotive force may exist between any of the elements of these plating systems when tied to aluminum that may not exist when tied to steel. The natural oxide coating on aluminum inhibits a tight adherency of the plated system. Without proper adherence, the nature of the galvanic couple therebetween may be increased or decreased due to the change in the current flow between the electrolyte of the galvanic couple and the particular metal forming the poles of the couple. For example, although stainless steel is much more noble than say copper, its effectiveness in the galvanic couple is considerably greater because of its high resistance to current flow through its interface with the electrolyte. Thus, plating systems for steel substrates must be carefully analyzed because of the unpredictable nature of applying such systems to aluminum and still achieve comparable results with respect to adherency and corrosion resistance.
In a separate path of technology and in an effort to cut down on the degree of electromotive potential between the elements of the system thereby hoping to improve lateral corrosion resistance, brass has been introduced to two known instances by the prior art for use in plating steel. In the first instance, the brass was constituted to contain a high proportion of zinc, about 70%, with copper maintained at about 30%. If this use of brass were to be applied to an aluminum substrate, zinc, being a highly reactive metal would become sacrificial and corrosion would proceed very rapidly laterally in the brass layer producing peeling and blistering under the decorative coating. In addition, the system would be limited to undesirable immersion coating techniques since consistent adherency of such a brass on aluminum by electroplating is not possible by the state of the art.
In the second instance, it was conceptualized that the electromotive potential of brass should be increased so that it is between steel and nickel; any galvanic couple created would be less and thereby slow down the rate of corrosion. This was implemented by increasing the copper content of the brass to 45-60%. If this concept were to be applied to an aluminum substrate, adherency of electroplating would still remain a severe problem and the galvanic couple between the increased-copper-brass and the aluminum would be greater than on steel; the brass would not be able to provide preferential cathodic protection to the decorative outer layers.
Accordingly, what is needed is a pretreatment which (1) provides for consistently good adherency of electroplated elements on aluminum, (2) reduces the galvanic couple potential between the aluminum and the most adjacent layer.